Florida accounts for more than 80% of the phosphate rock used in the United States and 30% of the world phosphate rock production. Each year more than 100 million tons of material is floated to generate 40-45 million tons of phosphate concentrate. Florida phosphate ore is known as matrix and consists of equal proportions of fluorapatite (a calcium phosphate containing fluorine), silica gangue and clays. Clays are separated from the matrix by washing and sizing leaving a phosphate and silica mixture. A three step process is used for separating phosphate from silica. In the first step, phosphate is separated from silica using a fatty acid and fuel oil mixture as a collector. The collector selectively coats the phosphate making it hydrophobic. When air is bubbled through the phosphate and silica suspension, the hydrophobic phosphate particles attach to the air bubbles and rise to the top where they are skimmed off. This method is known as rougher flotation. Silica is removed as a sink fraction and constitute rougher tailings. In the second step, the fatty acid and fuel oil layer are scrubbed off the phosphate particles by intense agitation and adjusting the pH to about 3. The treated material is washed until the pH is restored to a value of about 7. In the third step, fatty amine collectors are added to further reduce the amount of silica by rendering the surface of the silica hydrophobic leaving the phosphate hydrophilic. The floated silica particles constitute the froth whereas the phosphate particles sink and are referred to as the concentrate.
U.S. Pat. No. 3,164,549 discloses a process for beneficiating phosphate ores by froth flotation utilizing dodecyl benzene sulfonic acid. U.S. Pat. No. 4,034,863 discloses a process for beneficiating phosphate ores by froth flotation utilizing certain partial esters of a polycarboxylic acid. U.S. Pat. No. 4,172,029 discloses a process for beneficiating phosphate ores by froth flotation utilizing sulfonated aromatic compounds such as alkylated diphenyl ether sulfonate. However, the aforementioned patented flotation processes have not been of commercial value. The limited increased phosphate recovery effected by these processes is more than offset by the higher cost of the reagents relative to those used in the conventional process.
U.S. Pat. No. 4,719,009 discloses a process for concentrating zinc sulfide from complex sulfide ores containing siliceous gangue materials utilizing a depressant containing ether groups and metal sulfonate groups. The depressant is effective in an amount as low as 0.03 kg/ton of ore feed for the recovery of zinc sulfide concentrate. U.S. Pat. No. 4,719,009 is concerned with purifying zinc not increasing the yield of zinc. The sulfo-polyester acts to suppress silica. In phosphate flotation, on the other hand, the sulfo-polyester increases the phosphate yield without suppressing the silica. It is important to note that depressants are often found to be effective only in the treatment of certain specific ores due to the presence of salts in the water, the characteristics of ionic impurities associated with the siliceous gangue materials and other empirical factors, poorly understood.
The present inventors have unexpectedly determined that the addition of at least 0.6 kg/ton of a water dispersible, water dissipatable sulfo-polyester having a glass transition temperature (Tg) of 28.degree. C. to 60.degree. C. to a process for beneficiating a silicious phosphate ore by froth flotation increases the yield of phosphate.